In recent years, as DC power supplies for houses such as a solar power generator, a fuel cell, and an electric storage device become widespread, various circuits are proposed and provided as power-converting devices that convert outputs of these DC power supplies to AC. For example, in JP 2014-64431 A (Paragraphs [0002] to [0006], FIGS. 16 and 17, hereinafter referred to as Document 1) and in JP 4369425 B2 (hereinafter referred to as Document 2), power-converting devices (“multilevel power-converting device” in Document 1, “converter circuit” in Document 2) that generate AC outputs having voltage levels from a DC voltage supply are disclosed.
According to the description in Document 1, the power-converting device is a five-level inverter that outputs voltages of five levels, and includes two DC capacitors, two flying capacitors, and ten switching elements. The power-converting device outputs voltages of five levels by controlling the switching elements such that, in a state in which a DC voltage E is applied to a series circuit of the two DC capacitors, the voltage of each DC capacitor is E/2 and the voltage of each flying capacitor is E/4.
Incidentally, the power-converting device having the configuration described above includes, in modes having different output voltages, a mode in which the output voltage is generated by discharging the capacitor (flying capacitor). Therefore, the voltage of the capacitor is not always kept at a constant value. As a result, it is possible that the power-converting device will not generate a desired output voltage in the case where the fluctuation of the voltage of the capacitor is relatively large. Also, there is a problem in that, when a capacitor having a relatively large capacitance is used so as to suppress the fluctuation of the voltage of the capacitor, the size of the capacitor increases.